A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode contact can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission via coil 107 into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110.
Along the elongate axis of the electrode array 110 on its surface are multiple electrode contacts 112 that provide selective stimulation of the cochlea 104 e.g., by either monophasic or bi-phasic stimulation. The spacing between the electrode contacts 112 can be constant or variable. For example the electrode contacts 112 at the basal end of the electrode array 110 (closer to where the array enters the cochlea, e.g., through the oval window) may be more widely separated than those at the apical end of the electrode array 110.
Most existing cochlear implant stimulation coding strategies represent a sound signal by splitting it into distinct frequency bands and extracting the envelope (i.e., energy) of each of these bands. These envelope representations of the acoustic signal are used to define the pulse amplitude of stimulation pulses to each electrode. The number of band pass signals typically equals the number of stimulation electrodes, and relatively broad frequency bands are needed to cover the acoustic frequency range. Each electrode contact delivers electric stimulation signals to its adjacent neural tissue for a defined frequency band reflecting the tonotopic organization of the cochlea.
One neglected aspect in cochlear implant (CI) fittings is optimization of the frequency-band allocation of each electrode channel since the determination of place pitch using a psychoacoustic procedure is very time consuming. Generally, the perceived pitch is strongly correlated to the location of neural excitation along the cochlea (tonotopy). In normal hearing, the frequency-place map is logarithmic as defined by Greenwood (Greenwood, 1961). When several electrode channels excite similar regions of neuronal structures, more or less equal perceived pitch can be expected. If these electrode channels present different frequency-band signals to the same neurons, spectral- and temporal-confusion can result.
During electrode implantation surgery, the surgeon advances the electrode array through the oval window into the basal end of the scala tympani of the cochlea such that the distal tip of the electrode array reaches the apical region of the cochlea. But the high flexibility of the electrode array means that the surgical insertion procedure bears a significant risk of “fold-overs” of the electrode array within the cochlea. Particularly the apical tip of the electrode array may fold-over and the electrode contacts within the folded apical end will not reach the apical region of the cochlea, but instead will stimulate a more basal region of the cochlear which will elicit pitch confused hearing impressions in the implanted patient.
Such electrode fold-overs can be detected by imaging methods, e.g. computer tomographic scan (Grolman at al., “Spread of Excitation Measurements for the Detection of Electrode Array Foldovers: A Prospective Study Comparing 3-Dimensional Rotational X-ray and Intraoperative Spread of Excitation Measurements”, 2008). Such a complex and costly procedure is necessary because other measures such as eCAPs do not allow for distinguishing between a region where the neuronal nerve does not respond and situations where an electrode fold-over occurred; the measurement results do not allow any criteria to be established to differentiate between the two cases.